foam board insulation

Foam boards – rigid panels of insulation
– can be used to insulate almost any part of your home, from the roof
down to the foundation. They provide good thermal resistance and often add
structural strength to your home. Foam board insulation sheathing reduces
heat conduction through structural elements, like wood and steel studs.

Types of foam board

Molded expanded polystyrene foam board

Molded expanded polystyrene (MEPS) is a closed-cell material that can be
molded into many everyday items, such as coffee cups and shipping materials,
or into large sheets of foam board insulation. MEPS foam board insulation
is commonly known as beadboard.

To make beadboard, loose, unexpanded polystyrene beads containing liquid
pentane are mixed with a blowing agent and poured into an enclosed container.
The mixture is heated to expand the beads many times their original size.
The beads are then injected into a mold. Under more heat and pressure, they
expand to become foam blocks, which are shaped as needed.

The physical properties of MEPS foam board vary with the type of bead used.
It's manufactured at various densities, depending on the application. Beadboard
for roofing materials has to be dense enough to walk on without damage;
wall insulation foam boards are several times less dense than roof boards.
R-values range from 3.8 to 4.4 per inch (2.54 cm) of thickness.

MEPS foam board is available with a variety of facings.
Since spaces between the foam beads can absorb water, a vapor
retarder is necessary if water transmission through the insulation might
become a problem.

Extruded expanded polystyrene
foam board

Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation similar
to MEPS. To make it, the polystyrene pellets are mixed with various chemicals
to liquefy them. A blowing agent is then injected into the mixture, forming
gas bubbles. The foaming, thick liquid is then forced through a shaping
die. When cooled, the panel is cut as required. Foam densities are typically
1.5 pounds per cubic foot (0.21 kilograms per cubic meter).

XEPS is more expensive than MEPS. Like MEPS, the R-value depends upon the
density of the material and is generally about R-5 per inch. It's also much
more consistent in density and has a higher compressive strength than MEPS,
making it better suited for use on roofs or for wall panels. Extruded polystyrene
also has excellent resistance to moisture absorption.

Like MEPS, XEPS is available with a variety of facings and is also often
used as the insulation for SIPs and ICFs.

Polyisocyanurate
and polyurethane foam board

Polyisocyanurate or polyiso and polyurethane are very similar, closed-cell
foam insulation materials. Because both materials offer high R-values (R
5.6 to R 8) per inch of thickness, you can use a thinner foam board to achieve
the required thermal resistance. This can be an advantage if you have space
limitations.

Polyiso foam board insulation is available in a variety of compressive strengths.
Compressive strength refers to the ability of a rigid foam board to resist
deformation and maintain its shape when subjected to a force or load. Also,
polyiso remains stable over a wide temperature range (-100ºF to +250ºF).
This makes it good as roofing insulation. And when used with a laminated
aluminum foil facing, polyiso foam board provides an effective moisture
or vapor barrier.

These foam boards can also be used to make SIPs.

Installation

The maximum performance of foam board insulation depends heavily on proper
installation. Therefore, it's best to have a certified insulation installer
do it. If you'd like to have it installed professionally, you should do
the following:

Obtain written cost estimates from several contractors for the R-value
you need. Don't be surprised to find quoted prices for a given R-value
installation to vary by more than a factor of two.

Ask contractors about their air-sealing services and costs as well,
if needed.

To evaluate batt installation, you can measure batt thickness and
check for gaps between batts.

If you want to install it yourself, you should try to obtain instructions
and safety precautions from the manufacturer. Carefully follow these instructions.
You should also check your local building and fire codes.

Avoiding direct sunlight damage

Protect all types of foam insulation from direct sunlight. Over time, the
sun's ultraviolet rays can damage the insulation. For roofs, this is generally
done by applying a coating such as tar, acrylic, silicone, or rubberized
paint. You can also cover the foam with a rubber or plastic membrane, or
a layer of asphalt and roofing felt. Make certain you are using compatible
products. The solvents in some coatings dissolve certain plastics.

Avoiding potential moisture problems

In cold weather, warm inside air containing water vapor can get past the
wall finish and insulation, condensing inside the colder wall cavity. In
hot, humid climates the same thing can happen, just in the reverse direction.
Humid outdoor air in the summer can condense inside cool, air conditioned
wall cavities. If enough of this happens and the water cannot escape, wood
rot, mold, and other moisture-related problems can occur. For this reason,
building codes often require installing a vapor diffusion retarder on the
warmest side of the wall cavity.

Foam board insulation is commonly placed between the exterior finish (i.e.,
siding, brick) and the studs of exterior walls. To prevent air infiltration,
you should place rigid insulation boards tightly together and seal the seams
with tape or caulk. However, this practice may worry some builders in cold
climates since the foam board may act as a second vapor diffusion retarder.
Studies have shown, however, that condensation rarely occurs in these areas
unless something else is seriously wrong with the wall assembly (i.e., massive
uncontrolled air leakage into the walls from the house). If the assembly
is constructed correctly, the inside surface of the foam board stays warm
enough to keep water vapor in its gaseous state long enough for it to escape.

Avoiding potential insect problems

When insulating a foundation you need to consider that, although insects
don't eat foam board, they can easily tunnel through it. Insect burrows
reduce the R-value and structural integrity of the insulation. For these
reasons, some manufacturers treat their foam products with an insecticide,
usually a borate compound. Many building jurisdictions also mandate treating
the earth around the building with insecticides. These jurisdictions may
also want an inspection area several inches wide and all around the foundation
of a house kept bare of insulation board.

A better solution for below-grade walls in need of insulation is to install
the foam board over the interior of the basement walls rather than on the
exterior, which is more common. Interior applications prevent ground-dwelling
insects from finding the foam board at all, and they eliminate the need
for the bare inspection area. Insulating interior walls, however, requires
careful attention to moisture control.

Most jurisdictions also require installing a fire barrier over the interior
foam board. While this adds extra cost, the thermal performance of this
method is superior in most cases to the more common exterior foam board
application. This equates with a dollar savings in energy that can repay
you many times over for the additional cost of an interior application.
If you plan to convert a basement into a living space, there is almost no
additional cost.

Ensuring fire protection

Foam insulation is relatively hard to ignite, but when it is ignited, it
burns readily and emits a dense smoke containing many toxic gases. The combustion
characteristics of foam insulation products vary with the combustion temperatures,
chemical formulation, and available air.

Because of these characteristics, foams used for construction require a
covering as a fire barrier. One half-inch thick (1.27 cm) gypsum wallboard
is one of the most common fire barriers. Some building codes, however, do
not require an additional fire barrier for certain metal-faced, laminated
foam products. Check with your local building code/fire officials and insurers
for specific information on what is permitted in your area.